Single vs. Double Piston Rings

[Mastermind]

I may as well share my thoughts.

I would rather have both ring grooves filled. I'm of the opinion that the designer of the engine had a reason for using one or two rings, and I respect that decision. If the piston has two grooves.....I'll use two rings.

 

[old guy]

Yeah, I guess I'll leave em both on there.

John

 

[sunfish]

I have decided to try and replace my seized Husqvarna 353. I have been looking at Stihl, Husqvarna, Dolmar and Echo. The Husqvarna and Dolmar have single piston ring setups and the Stihl and Echo have double piston ring setups. I have bought a lot of weedeaters, blowers, etc and always look for double piston rings. What's the deal with Husky and Dolmar not using them? Would you expect to get less life out of them as compared to the Stihl or Echo?

You'd be better off to fix the 353... It's a damn good saw!

 

[edisto]

I may as well share my thoughts.

I would rather have both ring grooves filled. I'm of the opinion that the designer of the engine had a reason for using one or two rings, and I respect that decision. If the piston has two grooves.....I'll use two rings.

I have read a lot of speculation, but haven't seen any hard data either way. The consensus (although most of what I have read is for 2-strokes with a tuned pipe) is that if there are any gains in rpm by leaving a ring off, they are offset by losses at the low end, because compression will be lower and, to some degree, the charge in the crankcase will be contaminated and likely heated.

My rule of thumb is the same as yours...I count the number of grooves, and that's how many rings I use.

 

[motor head]

I have read a lot of speculation, but haven't seen any hard data either way. The consensus (although most of what I have read is for 2-strokes with a tuned pipe) is that if there are any gains in rpm by leaving a ring off, they are offset by losses at the low end, because compression will be lower and, to some degree, the charge in the crankcase will be contaminated and likely heated.

My rule of thumb is the same as yours...I count the number of grooves, and that's how many rings I use.

Very true I have seen a lot more carbon deposits in the bottom end of a saw with a single ring than one with a double ring piston.

 

[sunfish]

Very true I have seen a lot more carbon deposits in the bottom end of a saw with a single ring than one with a double ring piston.

I have a few single ring saws with tons of hours on them, no carbon build up in the bottoms at all...

 

[Mastermind]

Same here Don.

I've got zero issue with a single ring piston......but if there are two grooves they get rings in my saws.

Well 90% of the time. Sometimes the second ring gets in the way of the port work......the 51.4mm 372XPW top end comes to mind here. The ring end is right against the rear transfer port......it's real easy to end up with it riding in the port.....makes me nervous.

Brad is dead right about gaining RPM from tossing the second ring though.......I've seen it firsthand. I don't think that extra RPM is needed on a worksaw though.

 

[BVdog]

OK ready for a little old school stuff:
The heat energy in the piston is absorbed by the rings. The rings then transfer this heat into the cylinder walls and eventually into the surrounding air. Since the piston rings are in firm contact with both the piston and cylinder wall, they can 'take' heat energy from the piston and transfer this heat into the cylinder.
Above info from Riken Ring.. One of, if not the, largest ring producer in the world.

When you have less heat transfer with a single ring, the ring gets over-loaded and ring seal fades and with fading you get combustion blow-by(Motor Head above notice much more carbon buildup in the bottom end of single rings)... With blow-by you get power LOSS.. So, if you have blow-by (which you do) and you Lose power (which happens with blow-by) then you will now be over-fueling the cylinder, contaminating the intake charge (due the blow-by) which, in turn, lowers charge purity, which in turn, lowers power... So, now you have made your engine make less power BUT you are still feeding it with the same fuel (via carb jets) for the stock power.. So, now you have created a over-fueling (rich) condition inside the engine.. Rich conditions COOL the piston which is needed because of the ring over-load, and this same rich condition also aids in the power LOSS.. you can not make power with an overly rich condition...
So, it is a cycle that repeats over and over.. ring over-loads, charge density lessens, rich conditions occurs, power output is diminished, piston cools, ring seals normal again, back to stock power (or close to it) rinse, wash, and repeat..

It is VERY hard to seize an engine with a very rich condition... This is a very good reason to use a single ring. Less engine failure due to over-fueling. (some thought? many arguments?)
Remember you are taking compression readings on a NON running cooled engine at very low rpm's.

Detonation is another reason...you cannot pound between the 2 rings since there is only one.One of the MAIN signs of detonation is hammering between the rings (2)

Dyno Horsepower...short pulls to advertise HP..less drag with one ring

Most above info copied from RK Tek 2 stroke....Most is tech info on 2 strokes....food for thought and arguments.....

In the Motocross days we all ran Weisco single ring pistons...and changed them between races.

 

[blsnelling]

I made my mind up on this when I built the raffle saw that was in the MS460 buildoff. I did a lot of changes and testing during that build. I did nothing other than pull the bottom ring, went back out a few minutes later, and the saw made several hundred more RPMs in the cut. As far as cooling, I don't see the issue. Plenty of Huskies run a single ring. I've never had an issue with it. As far as compression, it has almost no effect at all. It's just another little piece of the pie. Some like it, some don't. That's cool too

 

[BVdog]

Conpression running vs cool compression test at very low rpm? Just something to think about. If you are racing that is one scenario but a working saw that has a piston designed for 2 rings ?? Something to think about


Sent from my iPhone using Tapatalk

 

[Alfa_Sam]

What about piston skirt slap? I would guess two rings would keep the piston centered in the bore better, reducing the chance of the piston skirt coming into contact with the cylinder wall.

 

[Mastermind]

No, the rings really won't stabilize the piston. The shape of the skirts have that job, and even the machine marks in the skirts play a role. They carry a layer of oil between the piston and jug. That's why when the machine marks have worn away, skirts will begin to break.

 

[CTYank]

What about piston skirt slap? I would guess two rings would keep the piston centered in the bore better, reducing the chance of the piston skirt coming into contact with the cylinder wall.

How else are you going to handle thrust forces, peaking at about mid-stroke because of rod angularity? Of course the oil film will prevent actual metal-to-metal.

The area involved is far greater than that of the ring interface, enabling much more heat transfer. See #28 above.

Piston slap was a problem with AL pistons in iron blocks (more clearance req'd) with no pin offset. As in MoPar hemis, until things got warmed up and clearance decreased.

 

[blsnelling]

The ring floats in the groove.

 

[edisto]

The ring floats in the groove.

Not during compression...if it did it wouldn't seal at all.

But you certainly would need more getting past the first ring than likely does to have the same effect on a second ring.

 

[blsnelling]

The ring is always floating in the groove. Combustion gases move behind the ring to force it against the cylinder wall. If the piston grew so much to press against the ring, it would likely seize.

 

[edisto]

The ring is always floating in the groove. Combustion gases move behind the ring to force it against the cylinder wall. If the piston grew so much to press against the ring, it would likely seize.

And those gasses are also exerting the same force onto the piston, and forcing the ring down onto the bottom of the groove. That is what I was talking about...if the ring isn't tight to the bottom of the groove, there won't be enough pressure to push it against the cylinder wall, and you'd have no seal.

We're probably talking about the same thing, it is just that "floating" is not the term I'd use for what happens to the ring. it makes me think of "flutter", and that is bad ju-ju.

 

[Mastermind]

Yeah.....I think we are all on the same page.

 

[blsnelling]

Yes we are

 

[edisto]

Yes we are

Literally and figuratively...unless this winds up on page 3.